Josie A. Galbraith

Invasive mammals have severely impacted oceanic islands, reducing or extirpating seabird populations and driving the collapse of nutrient flows with lasting ecosystem effects. We investigated these dynamics on contrasting subtropical oceanic islands in the South Pacific: Rangitāhua, a large island where goats, cats, and rats caused the extinction of seabird populations, and the small mammal‐free Meyer Islands, where seabird colonies remain dense. Using stable isotope analysis (δ ¹³ C and δ ¹⁵ N) of 408 contemporary and historical specimens spanning 169 years, across plants to invertebrates and birds, we tested hypotheses about seabird‐driven nutrient coupling across space and time. Spatially, we predicted stronger seabird‐driven nutrient enrichment on the mammal‐free Meyer Islands than on invaded Rangitāhua. Consistent with this, Meyer Islands' soils had higher phosphorus and lower pH, and terrestrial plants and herbivorous invertebrates were enriched in δ ¹³ C and δ ¹⁵ N relative to their Rangitāhua counterparts. In the nearshore marine system, δ ¹⁵ N values—but not δ ¹³ C—were elevated in macroalgae and herbivorous and omnivorous invertebrates around the Meyer Islands compared with those around Rangitāhua, indicating stronger seabird‐driven nutrient inputs to coastal food webs. By contrast, the lower values on Rangitāhua reflect slow seabird recovery more than 20 years after the eradication of invasive mammals. Temporally, we tested whether isotopic values in Rangitāhua plants and birds declined with seabird loss. Values for δ ¹³ C declined steadily from the mid‐19th century to the present, consistent with reduced seabird subsidies (and possibly volcanic activity), whereas δ ¹⁵ N values showed no long‐term change. Together, these spatial and temporal perspectives show that while seabird nutrient pathways can recover within decades on small islands, recovery on large, degraded islands can take much longer. Adjacent islets can provide recolonists but may not serve as true analogues. Restoring sea–land nutrient connectivity on large islands may require not only invasive mammal eradication but also active facilitation of seabird return.

Terrestrial birds were surveyed on Rangitāhua/Raoul Island in November 2023 closely following methods used in 1967, 1994, 1998, and 2008. Our counts sampled drier, lower-altitude coastal forest (300 m a.s.l.) on Denham Bay and Top Tracks, both of which had been sampled previously. Six indigenous species, pūweto/spotless crake (Zapornia tabuensis), pūkeko (Porphyrio porphyrio), red-crowned kākāriki (Cyanoramphus novaezelandiae cyanurus), kōtare/sacred kingfisher (Todiramphus sanctus), tūī (Prosthemadera novaeseelandiae), and welcome swallow/warou (Hirundo neoxena), and three exotic species, Eurasian blackbird (Turdus merula), song thrush (T. philomelos) and common starling (Sturnus vulgaris) were detected. In 5-min bird counts, tūī was the most commonly detected species followed by kākāriki, pūweto, common starling, Eurasian blackbird, song thrush, pūkeko, kotare, and welcome swallow. Kākāriki, tūī, and common starling were more numerous in the lower-altitude forest, while the numbers of pūweto, Eurasian blackbird, and song thrush were similar in both higher- and lower-altitude forests. Comparing the same subset of terrestrial birds, i.e. kākāriki, tūī, Eurasian blackbird, song thrush, common starling, and yellowhammer (Emberiza citrinella) surveyed previously, overall detection rates were significantly higher in 2023 than in 1967. The increase was largely due to significantly increased numbers of tūī and kākāriki. Since 1967, the composition of Rangitāhua’s terrestrial avifauna has shifted from exotic to indigenous dominance. Relative abundances of some terrestrial birds have also changed, especially pūweto, kākāriki, tūī, common starling, and song thrush. Kākāriki and pūweto were absent before 2008. Among the exotic birds, common starling and song thrush declined as proportions of total birds while Eurasian blackbird fluctuated. Feral goats (Capra hircus), rats (Rattus norvegicus and R. exulans), and cats (Felis catus) were respectively eradicated from Rangitāhua in 1972–84, 2002 and 2004. Increased indigenous dominance and changing proportions of species on Rangitāhua accord with findings elsewhere following pest mammal removal.

The Auckland Domain is the city’s oldest park and contains over 70 ha of contiguous, mature urban forest. Five-minute bird counts were made across one year within the domain forest in 2019 and 2020 and compared with counts conducted in 1987 and 1988, using the same methods and at the same survey sites, to investigate changes in the structure of the urban bird community. The abundance and species richness of native and introduced birds increased between the count years and there was structural change within the community driven by increases in the abundance of forest-adapted endemic species, tūī Prosthemadera novaeseelandiae, grey warbler Gerygone igata, and kererū Hemiphaga novaeseelandiae, and declines in generalist native species, silvereye Zosterops lateralis and fantail Rhipidura fuliginosa. Tūī showed the most profound increase in abundance between count years, reflecting regional conservation management of mainland and island forest habitats that benefit this highly mobile species. Increased abundance of eastern rosella Platycercus eximius and common myna Acridotheres tristis also altered community structure between count years, indicative of ongoing colonization by these exotic species in the Auckland region since their introduction to the North Island. The fact that both these species compete with native taxa for nest cavities within forests is of concern. Our results reinforce the need to manage and protect maturing urban forests to enhance native bird populations. Such actions will also support the recovery of native bird populations at a landscape scale.

There is an increasing disconnect between people and nature as we become more urbanised. Intensification in cities often results in a reduction of natural areas, more homogenised and manicured green spaces, and loss of biota. Compared to people in rural areas, urban dwellers are less likely visit natural areas and recognise and value biota. Reconnecting people with nature in the city not only benefits human mental and physical wellbeing but can also have positive effects on how people value biodiversity and act on conservation issues. However, in some contexts, the push to reconnect people with nature may have unintended negative outcomes on biodiversity, particularly if place-specific nature is not used in urban greening. In the current biodiversity crisis, using vegetation and green space design that is not reflective of the environmental context of a city can further disconnect residents, particularly Indigenous people, from their local environment and species, and further entrench extinction of experience and loss of environmental values. This disconnect can result in residents applying wildlife gardening practices, such as bird feeding, that are not specific to place, and benefit introduced species over indigenous species. Furthermore, cities are gateways for invasive species, and using species in greening projects that are not locally sourced has already left cities and their surrounding regions with a large weed legacy. Using place-specific nature and green space in cities can be less resource intensive, highly beneficial for biodiversity and give residents a unique sense of place. Rather than simply adding ‘more nature’ in cities, the messaging should be more complex, emphasising the need for urban greening to be context specific to avoid negative impacts on biodiversity and ecological and cultural services.

The global pet trade provides a pathway for introduced species to invade new environments. Most studies use trade data as an indirect proxy for propagule pressure exerted by the pet trade. Instead, we quantify the reported rate of loss of captive birds, assess factors that might influence this rate, simulate the survival and retrieval of birds and the overall cumulative propagule pressure exerted by pet birds on the environment. We used online listings of lost birds to estimate the propagule pressure that the pet trade exerts on the establishment of introduced bird species in Aotearoa–New Zealand. Listings from two popular websites were monitored daily for over 3.5 years, and information was recorded on the frequency, location, species composition and characteristics of the loss events. We investigated a range of factors that may influence the rate of loss events, such as season and human population size. We also developed a simulation approach to investigate the cumulative propagule pressure in Auckland, New Zealand's largest city. A total of 1205 birds and at least 33 species were reported lost nationwide during our monitoring period, 92% of which were parrots. We found that the reported loss rate was higher in areas of higher human population size and median income, and lower in the winter months. Simulation results predict that in any given month in Auckland there is an average of at least 491 escaped birds, including 136 potential breeding pairs, and for seven species the chance that at least one locality has a male/female pair at large exceeds 80%. Synthesis and applications. Online listings of lost pets provide an excellent source of data from which to identify species with high propagule pressure in specific localities. We identified escaped parrot species as a high‐risk invasion pathway, as they contribute to a high and consistent propagule pressure. A preventative approach, by banning the sale of these species, is the most appropriate pest management strategy for reducing the probability of establishment and potential impact.

Sugar-water bird feeding in residential backyards is increasingly popular, but its effects on wildlife are poorly understood. One concern is whether it results in maladaptive behaviour, such as reliance on artificial food or increased aggression due to increased density of visiting individuals. We studied sugar-water feeder-associated bird behaviour in two cities with different climates. We investigate whether season, city, or sugar concentration influenced bird foraging activity and aggressiveness. We then test whether feeder presence affected backyard bird composition. Birds were most aggressive and used sugar-water feeders most actively in winter, especially the omnivorous native tauhou (Zosterops lateralis). We also found city and seasonal differences in sugar-water feeder usage and aggression. Further, in Auckland, the city with the warmer climate, New Zealand’s largest nectarivorous species, tūī (Prosthemadera novaeseelandiae), was more likely to be aggressive at feeders with higher sugar concentrations but foraged longer at feeders with lower sugar concentrations. Neither feeder presence nor sugar concentration influenced garden bird species richness or abundance. We discuss the effects of sugar-water feeding on bird behaviour at the global and local scale and suggest future study directions.

As cities expand, understanding how urbanization affects biodiversity is a key ecological goal. Yet, little is known about how host‐associated microbial diversity responds to urbanization. We asked whether communities of microbial (bacterial and fungal) in floral nectar and sugar‐water feeders and vectored by nectar‐feeding birds—thus forming a metacommunity—differed in composition and diversity between suburban and rural gardens. Compared to rural birds, we found that suburban birds vectored different and more diverse bacterial communities. These differences were not detected in the nectar of common plant species, suggesting that nectar filters microbial taxa and results in metacommunity convergence. However, when considering all the nectar sources present, suburban beta diversity was elevated compared to rural beta diversity due to turnover of bacterial taxa across a plant species and sugar‐water feeders. While fungal metacommunity composition and beta diversity in nectar were similar between suburban and rural sites, alpha diversity was elevated in suburban sites, which mirrored the trend of increased fungal alpha diversity on birds. These results emphasize the interdependence of host, vector, and microbial diversity and demonstrate that human decisions can shape nectar microbial diversity in contrasting ways for bacteria and fungi.

Garden bird sugar water feeding is increasingly popular worldwide, but little is known about its effects on bird health and associated diseases. There is a concern that feeding stations can accumulate pathogens and facilitate pathogen transmission between individuals, resulting in adverse effects on body condition of visiting birds. We tested the effects of sugar water feeding in urban New Zealand backyards by sampling target species for multiple infections and comparing bird body condition. For this, we compared backyards with and without sugar water feeders and again compared existing sugar water feeders with various sugar concentrations in two cities and in two seasons. Birds caught in gardens with sugar water feeders had poorer body condition; however, birds had better body condition in the city with the warmer climate (Auckland), during summer, and in gardens with high (≥20%) sugar concentration in sugar water feeders in winter. All screening tests for Chlamydia psittaci and Salmonella spp. returned negative results. Avian poxvirus prevalence in tauhou ( Zosterops lateralis) was four times higher in the city with a warmer climate. The likelihood of lice infection in tauhou was lower in gardens with feeders, in the warmer city, in summer, and at feeders with higher sugar concentrations. In tūī ( Prosthemadera novaeseelandiae), the likelihood of lice infection decreased with an increase in sugar concentration. Coccidia infection was 4.25 times higher in tauhou in gardens with feeders. Despite the identified risks associated with sugar water feeding, there appear to be potential benefits for native nectarivorous birds, specifically in winter.

The practice of garden bird feeding is a global phenomenon, involving millions of people and vast quantities of food annually. Many people engage in the practice of feeding assuming that birds gain some benefit from the food they provide, yet recent studies have revealed the potential for detrimental impacts as well. However, there is still a paucity of information on the impacts of feeding, including the ubiquity of these impacts among and within feeder-visiting species. Consistency in feeder use among birds is likely an important determinant of this. Individual birds and species that make frequent use of feeders are more likely to experience both the benefits and detrimental impacts of supplementary food. We investigated patterns of feeder use by garden birds visiting experimental feeding stations in Auckland, New Zealand, with the specific aim of determining whether use of supplementary food was consistent or variable among individuals and species. We used camera traps as well as Radio Frequency Identification (RFID) technology to examine intra- and interspecific feeder visitation patterns and to discern species associations. Eleven bird species were detected using feeding stations, however, two introduced species (house sparrow Passer domesticus and spotted dove Streptopelia chinensis) dominated visitation events. These species were present at feeders most frequently, with the largest conspecific group sizes. Significant associations were detected among a number of species, suggesting interspecific interactions are important in determining feeder use. We also found within-species differences in feeder use for all focal species, with individual variation greatest in house sparrows. Furthermore, season had an important influence on most visitation parameters. The observed individual and species-specific differences in supplementary food resource use imply that the impacts of garden bird feeding are not universal. Crucially, particularly given the avifaunal context in New Zealand, resource dominance by introduced species could have potential negative outcomes for native species conservation in cities.

Intentional feeding of wild birds in gardens or backyards is one of the most popular forms of human-wildlife interactions in the developed world, especially in urban environments. Scale and intensity of bird feeding are enormous with mainly birdseed consumed daily by a range of species. This represents a subsidy to natural diets of birds attracted to the feeders and typically involves novel dietary components. Yet, relatively little is known about how it influences the behavior and ecology of the species visiting feeders. In part, research has been hampered by logistical difficulties of working in urban areas but studies have demonstrated powerful influences on behavior and phenology of avian breeding, the spread of disease, and the structure of avian communities. Here, we compare bird feeding between Northern and Southern Hemispheres as a means of exploring how similarities and differences in avian responses might inform knowledge of this global urban phenomenon. We start by tracing its origins to north-western Europe and how its expansion has occurred before considering how geographical differences in feeding practices and attitudes map onto bird feeding ‘on the ground’. We explore some of the major emerging themes of recent interest, including why citizens are motivated to feed birds, whether birds become fully dependent on food supplements, the role of feeding in avian disease transmission, and how feeding changes urban bird communities. By proposing that scientists work in collaboration with the public providing food to birds, we pose key research questions that need to be answered urgently and suggest accompanying experimental approaches to do so. These approaches are essential if we are to improve our understanding of how bird feeding shapes the behavior, ecology, movements, and community structure of urban birds. Our hope is that through such citizen science we will be able to provide advice as to location-relevant practices that should maximise benefits to both urban biodiversity and human wellbeing, and minimise potential adverse impacts. We demonstrate that bird feeding is important for urban biodiversity conservation, community engagement, and in establishing personal connections with nature and their associated benefits.